<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <div id="app">app</div>
</body>
<script src="https://unpkg.com/react@16/umd/react.production.min.js" crossorigin></script>
<script src="https://unpkg.com/react-dom@16/umd/react-dom.production.min.js" crossorigin></script>
<script src="https://unpkg.com/babel-standalone@6/babel.min.js"></script>
<script type="text/babel">
  let msg = 'ni hao'

  function btnClick() {
    msg = 'hello world'
    render()
  }

  function render() {
    ReactDOM.render(
      <div >
        <h2>{msg}</h2>
        <button onClick={btnClick}>change</button>
      </div>,
      document.getElementById('app')
    )
  }
  
  render()
</script>
</html>